E-liquid or more commonly called Vape Juice usually comes in 3 main types, The Main differences being the Consistancy of the E-liquid or the type of nicotine Used and its Important to choose the right E-liquid type for the Vape Kit you are Using so that the coil is able to function properly, If the liquid is too thick the coil may burn out quickly if its too thin it may cause the tank to leak. 
1-  50vg 50pg which is the perfect juice for starter kits as it gives a good balance of flavour and throat hit and isnt too thick for coils on ecig starter kits.
2- Nic Salts, Most are 50/50 but the difference with Nic salts is that the Nicotine Base used is slightly different and enables the user to use a higher nicotine level but with a smoother throat hit. Nic Salts are great in Pod Systems and Starter Kits. 
3- High Vg- These liquids are Higher in Vg meaning the liquid is thicker and will produce more vapour and a smooth throat hit, High Vg E-liquid is Perfect for high powered Sub Ohm Kits, We would not recomend using it in Normal Starter Kits.